Can I assign "U" for "undo"?
This is because the shortcut you re-assigned only applies to the Non-Linear Animation Editor.

To find all conflicting shortcuts, search based on key binding instead:



There are U shortcuts in the 3D view (both in object mode and edit mode) which call the call_menu() operator. This is different from the NLA editor, which just runs nla.make_single_user() directly when called since it has no options.

Disable or re-assign the conflicting shortcuts, then it works as expected.